Method and apparatus for avoidance of phrenic nerve stimulation during cardiac pacing

ABSTRACT

A cardiac rhythm management device in which an accelerometer is used to detect diaphragmatic or other skeletal muscle contraction associated with the output of a pacing pulse. Upon detection of diaphragmatic contraction, the device may be configured to automatically adjust the pacing pulse energy and/or pacing configuration.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/966,818, filed on Sep. 28, 2001, the specification of which isincorporated herein by reference.

FIELD OF THE INVENTION

This invention pertains to methods and apparatus for cardiac rhythmmanagement. In particular, the invention relates to methods andapparatus for cardiac pacing with electrical stimulation.

BACKGROUND

Cardiac rhythm management devices are implantable devices that provideelectrical stimulation to selected chambers of the heart in order totreat disorders of cardiac rhythm. A pacemaker, for example, is acardiac rhythm management device that paces the heart with timed pacingpulses. The most common condition for which pacemakers are used is inthe treatment of bradycardia, where the ventricular rate is too slow.Atrio-ventricular conduction defects (i.e., AV block) that are permanentor intermittent and sick sinus syndrome represent the most common causesof bradycardia for which permanent pacing may be indicated. Iffunctioning properly, the pacemaker makes up for the heart's inabilityto pace itself at an appropriate rhythm in order to meet metabolicdemand by enforcing a minimum heart rate.

Pacemakers are usually implanted subcutaneously or submuscularly on apatient's chest and have leads threaded intravenously into the heart toconnect the device to electrodes used for sensing and pacing. Leads mayalso be positioned on the epicardium by various means. A programmableelectronic controller causes the pacing pulses to be output in responseto lapsed time intervals and sensed electrical activity (i.e., intrinsicheart beats not as a result of a pacing pulse). Pacemakers senseintrinsic cardiac electrical activity by means of internal electrodesdisposed near the chamber to be sensed. A depolarization wave associatedwith an intrinsic contraction of the atria or ventricles that isdetected by the pacemaker is referred to as an atrial sense orventricular sense, respectively. In order to cause such a contraction inthe absence of an intrinsic beat, a pacing pulse (either an atrial paceor a ventricular pace) with energy above a certain pacing threshold isdelivered to the chamber via the same or different electrode used forsensing the chamber.

Electrical stimulation of the heart through the internal electrodes,however, can also cause unwanted stimulation of skeletal muscle. Theleft phrenic nerve, which provides innervation for the diaphragm, arisesfrom the cervical spine and descends to the diaphragm through themediastinum where the heart is situated. As it passes the heart, theleft phrenic nerve courses along the pericardium, superficial to theleft atrium and left ventricle. Because of its proximity to theelectrodes used for pacing, the nerve can be stimulated by a pacingpulse. The resulting involuntary contraction of the diaphragm can bequite annoying to the patient, similar to a hiccup.

SUMMARY OF THE INVENTION

The present invention is a cardiac rhythm management device that isconfigured to detect when unwanted stimulation of skeletal muscle suchas the diaphragm occurs during pacing by sensing the resultingacceleration imparted to the device housing. Signal processingtechniques may be used to distinguish the acceleration that results fromskeletal muscle contraction from that due to cardiac contraction (i.e.,heart sounds). If skeletal muscle contraction occurs, the device maythen decrease the pacing pulse energy. When adjusting the pacing pulseenergy, a capture verification test may be performed by sensing evokedpotentials during pacing in order to ensure that pacing pulses haveadequate energy to stimulate the heart. In another embodiment, thepacing configuration used for outputting pacing pulses may be modifiedto result in a pacing vector less likely to cause skeletal musclecontraction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram of a cardiac rhythm management deviceconfigured for evoked potential sensing.

FIG. 2 illustrates an exemplary switching circuit.

FIG. 3 illustrates an exemplary signal processing module.

FIG. 4 is a flow diagram illustrating an exemplary method forautomatically the adjusting the pulse energy and/or pacing configurationin accordance with accelerometer signals.

DETAILED DESCRIPTION

The present invention is applicable to all types of cardiac rhythmmanagement devices that have a pacing functionality includingbradycardia pacing, anti-tachycardia pacing, and cardiacresynchronization pacing. After a description of the basic hardwarecomponents and operating modes of a pacemaker, exemplary embodiments ofthe invention will be set forth.

1. Device Description

A block diagram of a multi-site pacemaker having an atrial and twoventricular pacing channels is shown in FIG. 1. The control unit of thepacemaker is made up of a microprocessor 10 communicating with a memory12 via a bidirectional data bus, where the memory 12 typically comprisesa ROM (read-only memory) for program storage and a RAM (random-accessmemory) for data storage. The control unit could be implemented by othertypes of logic circuitry (e.g., discrete components or programmablelogic arrays) using a state machine type of design, but amicroprocessor-based system is preferable. The control unit is capableof operating the pacemaker in a number of programmed modes where aprogrammed mode defines how pacing pulses are output in response tosensed events and expiration of time intervals. Bradycardia pacing modesrefer to pacing algorithms used to pace the atria and/or ventricles whenthe intrinsic atrial and/or ventricular rate is inadequate due to, forexample, AV conduction blocks or sinus node dysfunction. Such modes mayeither be single-chamber pacing, where either an atrium or a ventricleis paced, or dual-chamber pacing in which both an atrium and a ventricleare paced. Another type of pacing is anti-tachycardia pacing where theheart is paced competitively in order to slow an abnormally fast rhythm.Pacemakers can also be employed to improve the coordination of cardiaccontractions by timed pacing of selected chambers or sites, termedcardiac resynchronization therapy. Additional sensing of physiologicaldata allows the pacemaker to change the rate at which it paces the heartin accordance with some parameter correlated to metabolic demand,referred to as rate-adaptive pacing. One such parameter is the activitylevel of the patient. In the device of FIG. 1, an accelerometer 330senses accelerations imparted to the device housing brought about bychanges in the patient's physical activity. A telemetry interface 80 isalso provided for communicating with an external programmer.

The pacemaker has an atrial sensing/pacing channel comprising ringelectrode 43 a, tip electrode 43 b, sense amplifier 41, pulse generator42, and an atrial channel interface 40 which communicatesbidirectionally with a port of microprocessor 10. The device also hastwo ventricular sensing/pacing channels that include ring electrodes 23a and 33 b, tip electrodes 23 b and 33 b, sense amplifiers 21 and 31,pulse generators 22 and 32, and ventricular channel interfaces 20 and30. The electrodes are electrically connected to the device by means ofa lead. The ring and tip electrode associated with each channel can beused for bipolar sensing or pacing or, as described below, differentelectrodes can be connected to each channel through a switching circuit70 to result in different unipolar sensing or pacing vectors. Thesensing circuitry of the pacemaker generates atrial and ventricularsense signals when voltages sensed by the electrodes exceed a specifiedthreshold. The pacemaker also has an evoked response sensing channelthat comprises an evoked response channel interface 50 and a senseamplifier 51. The channel interfaces include analog-to-digitalconverters for digitizing sensing signal inputs from the sensingamplifiers, registers that can be written to for adjusting the gain andthreshold values of the sensing amplifiers, and, in the case of theventricular and atrial channel interfaces, registers for controlling theoutput of pacing pulses and/or changing the pacing pulse amplitude.

The controller 10 controls the overall operation of the device inaccordance with programmed instructions stored in memory. The controller10 interprets sense signals from the sensing channels and controls thedelivery of paces in accordance with a pacing mode. The controller isalso interfaced to a switching circuit 70 through which the electrodesare connected to the sense amplifiers and pulse generators. Thecontroller is thus able to connect the amplifiers and/or pulsegenerators to selected tip or ring electrodes of any of thesensing/pacing channels that connect through the switching circuit 70.Each sense amplifier amplifies the voltage difference between twoinputs, and the inputs may be selected from any of the tip or ringelectrodes or the pacemaker case or can 60, which is also electricallyconnected to the switching circuit. The device has the capability ofconnecting a pulse generator such that a pacing voltage pulse appearsacross any of the tip or ring electrodes or across an electrode and thecan 60. A particular set of electrodes and one or more pulse generatorsused to output pacing pulses is referred to herein as a pacingconfiguration.

The switching circuit 70 may be implemented as an array of MOSFETtransistors controlled by outputs of the controller 10. FIG. 2 shows aportion of a basic exemplary switching circuit. In this circuit, a pairof MOSFET transistors Q1 and Q2 along with inverter INV form adouble-pole switch that switches one of the inputs to amplifier 51between ring electrode 23 a and 33 a in accordance with a control signalCS from the controller. The other input is shown as being connected tocan 60, but in other embodiments it may also be switched to one of theelectrodes by means of the switching circuit. In a more complicatedversion of the same basic pattern, the switching circuit 70 may be ableto switch the amplifier inputs or pulse generator outputs to any of thetip or ring electrodes of the sensing/pacing channels or to the can 60.

2. Capture Verification

In order for a pacemaker to control the heart rate in the mannerdescribed above, the paces delivered by the device must achieve“capture,” which refers to causing sufficient depolarization of themyocardium that a propagating wave of excitation and contraction result(i.e., a heart beat). A pacing pulse that does not capture the heart isthus an ineffective pulse. This not only wastes energy from the limitedenergy resources (battery) of pacemaker, but can have deleteriousphysiological effects as well, since a demand pacemaker that is notachieving capture is not performing its function in enforcing a minimumheart rate. A number of factors can determine whether a given pacingpulse will achieve capture including the energy of the pulse, which is afunction of the pulse's amplitude and duration or width, and theintegrity and physical disposition of the pacing lead.

A common technique used to determine if capture is present during agiven cardiac cycle is to look for an “evoked response” immediatelyfollowing a pacing pulse. The evoked response is the wave ofdepolarization that results from the pacing pulse and evidences that thepaced chamber has responded appropriately and contracted. By detectingthe evoked P-wave or evoked R-wave, the pacemaker is able to detectwhether the pacing pulse (A-pulse or V-pulse) was effective in capturingthe heart, that is, causing a contraction in the respective heartchamber. In order for a pacemaker to detect whether an evoked P-wave oran evoked R-wave occurs immediately following an A-pulse or a V-pulse, aperiod of time, referred to as the atrial capture detection window orthe ventricular capture detection window, respectively, starts after thegeneration of the pulse. Sensing channels are normally renderedrefractory (i.e., insensitive) for a specified time period immediatelyfollowing a pace in order to prevent the pacemaker from mistaking apacing pulse or after potential for an intrinsic beat. This is done bythe pacemaker controller ignoring sensed events during the refractoryintervals, which are defined for both atrial and ventricular sensingchannels and with respect to both atrial and ventricular pacing events.Furthermore, a separate period that overlaps the early part of arefractory interval is also defined, called a blanking interval duringwhich the sense amplifiers are blocked from receiving input in order toprevent their saturation during a pacing pulse. If the same sensingchannels are used for both sensing intrinsic activity and evokedresponses, the capture detection window must therefore be defined as aperiod that supercedes the normal refractory period so that the sensingcircuitry within the pacemaker becomes sensitive to an evoked P-wave orR-wave.

Capture verification can be performed by delivering a pacing pulse andattempting to sense an evoked response using either the same ordifferent electrodes used for pacing. In an exemplary embodiment shownin FIG. 1, a capture verification test is performed using a dedicatedevoked response sensing channel that includes a sense amplifier forsensing an evoked response generated after a pacing pulse is delivered.The amplifier input of the evoked response sensing channel is switchedvia switching circuit 70 to selected electrodes of the sensing/pacingchannels before the capture verification test is performed. Afterswitching the input of the evoked response sensing channel to theselected electrodes, a pacing pulse is output and an evoked response iseither detected or not, signifying the presence or loss of capture,respectively. Although the same electrodes can be used for pacing andevoked response detection during a capture verification test, the inputof the evoked response sensing channel preferably is switched toelectrodes of another sensing/pacing channel. The particular electrodesused for evoked response detection can be selected in accordance withwhich electrodes produce a sensing vector that most easily senses anevoked response due to the pacing electrodes. The sense amplifier of theevoked response sensing channel is then blanked during the captureverification test for a specified blanking period following a pacingpulse output by the tested sensing/pacing channel. The blanking periodis followed by a capture detection window during which an evokedresponse may be sensed by the evoked response sensing channel. In anexemplary embodiment, the blanking period may be approximately 10 ms,and the width of the capture detection window may range from 50 to 350ms.

3. Detection and Avoidance of Skeletal Muscle Stimulation

As noted above, pacing pulses can stimulate the phrenic nerve and causecontraction of the diaphragm. It is also possible for unipolar pacingconfigurations to produce a pacing vector that stimulates the pectoralmuscles overlying the internal electrodes, resulting in so-called pockettwitch. Both types of skeletal muscle stimulation can be very annoyingto a patient. Abrupt contractions of either the pectoral muscles or thediaphragm will impart an acceleration to the implanted housing of thepacemaker. In order to detect whether pacing pulses are producing suchunwanted muscle contractions, the controller 10 can be configured to usethe accelerometer 330 to sense any accelerations experienced by thedevice housing that coincide with the output of a pacing pulse.Contraction of the heart and the resulting heart sounds, however, canalso impart an acceleration to the device housing that coincides with apacing pulse. In order to distinguish this type of acceleration fromthat due to skeletal muscle contraction, signal processing techniquescan be used. For example, the acceleration signal produced by theaccelerometer 330 during an intrinsic heartbeat or a low-energy pacedheartbeat with no accompanying skeletal muscle contraction can berecorded and processed to produce a cardiac signature in either the timeor frequency domain that can be compared with accelerometer signalsobtained while pacing. An accelerometer signal obtained during a pacingtime window starting with a pacing pulse (e.g. 10 to 100 millisecondspost-pace) that contains frequency components different from that of thecardiac signature or is uncorrelated with the cardiac signature in thetime domain, for example, can be assumed to be due to skeletal musclecontraction. In an alternative embodiment, the patient may undergoclinical testing in which diaphragmatic contractions or pocket twitchesare intentionally produced by varying the pacing energy and/or pacingconfiguration in order to obtain a skeletal muscle contraction signaturefrom an accelerometer measurement during the pacing time window. Theskeletal muscle signature can then be compared in either the time orfrequency domain with accelerometer measurements taken within the pacingtime window to determine if the acceleration is due to skeletal musclecontraction.

FIG. 3 illustrates a signal processing module 300 for performing thecomparison between an accelerometer signal and a signature as describedabove. The module 300 may be incorporated into the controller either ascode executed by the microprocessor or as one or more discrete hardwarecomponents and compares the accelerometer signal AS obtained during thepacing time window with a signature signal SS. As noted above, such acomparison may be performed in either the time domain or the frequencydomain. In a particular embodiment, the module 300 may be a matchedfinite impulse response filter that performs a cross-correlation betweenthe accelerometer signal AS and the recorded signature signal SS. Therecorded signature signal SS is represented in that case by the filtercoefficients of the matched filter (i.e., the impulse response of thefilter corresponds to a time-reversed version of the signature signalSS).

Once it is determined that unwanted skeletal muscle contractions areoccurring with pacing pulses, the controller may be further programmedto make adjustments in the operation of the device. In one embodiment,capture verification tests can be performed as the pacing pulse energyis reduced until a pacing pulse energy is found that both achievescapture and produces no skeletal muscle contraction. In otherembodiments, the pacing configuration can be varied. For example,different pacing vectors can be tried by switching the output of a pulsegenerator to different electrodes with the switching circuit 70.Switching from a unipolar to a bipolar pacing configuration, forexample, may prevent pacing pulses from causing pectoral musclecontraction. Other pacing configurations with different pacing vectorsmay be less likely to stimulate the phrenic nerve. In the case ofmulti-site pacing, different pacing configurations using fewer ordifferent pacing sites may also be tried.

FIG. 4 is a diagram illustrating the steps performed by an exemplarysystem for automatically adjusting device operation as it could beimplemented in software executed by the controller and/or with dedicatedhardware components. At step S1, the system waits for the pacingalgorithm to output a pacing pulse and then records the accelerometersignal during a specified pacing time window after the pace (e.g. 10 to100 milliseconds post-pace). A time domain or frequency domaincomparison is then performed between the accelerometer signal and eithera cardiac or skeletal muscle signature at step S3. At step S4, adetermination is made as to whether the accelerometer signal representsa skeletal muscle contraction and the system returns to step S1 if not.Otherwise, the pulse energy is decreased by an amount X and/or thepacing configuration is altered at step S5, and the system waits foranother pace to be output at step S6. At step S7, a capture verificationtest is performed. If capture is achieved the system returns to step S1.If the pace fails to achieve capture, the pulse energy is increased bysome fraction of X (and/or the pacing configuration is altered in somemanner that reverses the effect the alteration performed at step S5) andthe system returns to step S6. Thus, in this embodiment, no furtheracceleration sensing is performed by steps S1 through S4 until the pulseenergy and/or pacing configuration is adequate to achieve capture.

Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

1. A cardiac rhythm management device, comprising: circuitry for sensingand pacing a heart chamber; an accelerometer for sensing accelerationsand generating an accelerometer signal in accordance therewith;circuitry for comparing the accelerometer signal obtained during apacing time window subsequent to the output of a pacing pulse with asignature to determine if skeletal muscle contraction has occurred. 2.The device of claim 1 wherein the signature represents an accelerationsignal generated after an intrinsic heartbeat.
 3. The device of claim 1wherein the signature represents an acceleration signal generated aftera paced beat that produces diaphragmatic contraction.
 4. The device ofclaim 1 wherein the signature represents an acceleration signalgenerated after a paced beat that produces a pocket twitch.
 5. Thedevice of claim 1 wherein the comparison between the accelerometersignal and the signature is a frequency domain comparison.
 6. The deviceof claim 1 wherein the comparison between the accelerometer signal andthe signature is a time domain correlation.
 7. The device of claim 1further comprising circuitry for decreasing the pacing pulse energy by aspecified amount if skeletal muscle contraction has been detected. 8.The device of claim 7 further comprising circuitry for performing acapture verification during a subsequent pacing pulse after the pulseenergy has been decreased, the capture verification test being performedby sensing whether an evoked response occurs during a capture detectionwindow following the output of the pacing pulse, and increase the pacingpulse energy if no capture was achieved.
 9. The device of claim 1further comprising circuitry for altering a selected pacingconfiguration if skeletal muscle contraction has been detected.
 10. Thedevice of claim 7 further comprising circuitry for performing a captureverification during a subsequent pacing pulse after the pulse energy hasbeen decreased, the capture verification test being performed by sensingwhether an evoked response occurs during a capture detection windowfollowing the output of the pacing pulse, and to alter a selected pacingconfiguration if no capture was achieved.
 11. The device of claim 1further comprising a signal processing module for comparing theaccelerometer signal with the signature.
 12. The device of claim 11wherein the signal processing module performs a cross-correlationbetween the accelerometer signal and the signature.
 13. A method foroperating a cardiac rhythm management device, comprising: outputtingpacing pulses to a heart chamber; and, comparing an accelerometer signalobtained during a pacing time window subsequent to the output of apacing pulse with a signature to determine if skeletal musclecontraction has occurred.
 14. The method of claim 13 wherein thesignature represents an acceleration signal generated after an intrinsicheartbeat.
 15. The method of claim 13 wherein the signature representsan acceleration signal generated after a paced beat that producesdiaphragmatic contraction.
 16. The method of claim 13 wherein thesignature represents an acceleration signal generated after a paced beatthat produces a pocket twitch.
 17. The method of claim 13 wherein thecomparison between the accelerometer signal and the signature is afrequency domain comparison.
 18. The method of claim 13 wherein thecomparison between the accelerometer signal and the signature is a timedomain correlation.
 19. The method of claim 13 further comprisingdecreasing the pacing pulse energy by a specified amount if skeletalmuscle contraction has been detected.
 20. The method of claim 19 furthercomprising performing a capture verification during a subsequent pacingpulse after the pulse energy has been decreased, the captureverification test being performed by sensing whether an evoked responseoccurs during a capture detection window following the output of thepacing pulse, and increasing the pacing pulse energy if no capture wasachieved.